Photopolymerizable mixtures and recording materials which are produced from it are predominantly used for the production of printing plates, and relief pictures, as well as resist materials used in the electronic industry for the production of printed circuits.
In order to comply with the requirements of practice, such materials must, particularly if they are to be used as resist materials, exhibit a number of properties which are often very difficult to combine.
It is, for example, required of resist materials, which are to be washed out and where the remaining coating is to be subsequently removed, that the unexposed areas can be quickly washed out with aqueous-alkaline solutions, whereas the exposed areas must be completely resistant towards these developer solutions. During the subsequent processing it must also be possible to remove the exposed areas quickly and without residue using stronger alkaline aqueous solutions. This differentiated solubility behavior of the unpolymerized and polymerized areas is essentially based on the attack of the aqueous-alkaline developer solution on the soluble binding agent in the unexposed areas, in such a way that the hydrophobic monomer, enclosed in the binding agent, is washed out together with the binding agent. In the exposed areas, crosslinking of the monomer protects the binding agent, which is inherently soluble in the developer solution from the hydrophilic action of developer solution.
If the exposed areas are not sufficiently crosslinked, the processing latitude is unacceptably decreased. In addition, the exposed areas may be attacked by the hydrophilic alkaline etching solution and thus perform inadequately as a resist. The more hydrophobic the exposed regions are the better is the quality of the washed out relief. A slight swelling of the polymerized areas in the hydrophilic developer can lead to distortion of very fine line structure and can thus impair reproduction quality. Such quality impairments cannot be reversed even after drying. In addition, the swelling prevents the formation of straight resist sidewalls.
For the production of resist materials which are to be developed and stripped in aqueous-alkaline solutions, it is thus customary to use monomers which are sufficiently hydrophobic that no undesirable swelling occurs during processing, and which in addition strongly crosslink the binding agent in the exposed areas.
It is a disadvantage that the more hydrophobic the photopolymerized layer remaining on the substrate is after the development process, once it has fulfilled its purpose as resist, the more difficult, slow and incomplete is the removal with the hydrophilic stripping solution. Attempts to improve the stripping properties by decreasing the network density, for example by lowering the concentration of the monomer, have shown that the processing latitude is inadmissibly decreased. Addition of low-molecular weight materials, e.g., plasticizers improves the stripping behavior, but this causes a deterioration of the mechanical properties of the material, particularly in tenting applications. In addition, plating baths may be contaminated by extraction of the plasticizers.
Since the alkaline stripping solution cannot directly attack the polymerized areas, the stripping of the known aqueous-alkaline processible materials occurs by decreasing the adhesion between polymerized layer and metallic base by action of the stripping solution. This causes the polymerized layer to peel off in large flakes.
These "peeled off" resist flakes frequently cause interruptions within the stripping machines by clogging pipes, pumps, and the like.
However, it is much more disadvantageous that particularly in the areas of fine line elements or in the areas which were excessively overplated, the adhesion between metallic base and photopolymerized layer is not sufficiently reduced in the time provided by the processing machine. Thus, the printed circuit boards are not completely stripped despite a subsequent spraying process after passing through the stripping machine. This impairs the performance of the subsequent operating cycles and thus also the quality of the printed circuits.
For a good stripping behavior it is thus necessary that the stripping not only occurs completely, but also takes place within the short period of time provided for by the processing machine,
From the DE-PS No. 26 11 577 aqueous-alkaline processible photopolymerizable resist materials have become known in which the photopolymerized areas which serve as etching and/or plating resist, are soluble in the aqueous-alkaline stripping solution. Such resist materials can be stripped naturally without any residues. It is, however, a disadvantage that, with regard to the constantly increasing demands of environmental protection, the elimination of the stripping solution, which exhibits a high concentration of dissolved harmful materials, causes considerable difficulties.
It is thus the task of the invention to describe photopolymerizable mixtures and recording materials produced from them which, compared to known aqueous-alkaline processible recording materials, distinguish themselves by improved stripping behavior, which does not impair other properties of the material, such as processing latitude, reproduction quality of fine line elements, or mechanical properties, and with which the stripping solution or the plating baths are not contaminated in an admissible manner by dissolved harmful materials.
This task is achieved by photopolymerizable mixtures and recording materials produced from them of the type mentioned in the beginning, which contain at least one photopolymerizable monomer according to the present invention.